Chucks for lathes in different designs are well-known from the state of the art. For clamping round workpieces, the state of the art normally uses a three-jaw chuck with lathes and a so-called four jaw chuck for clamping square workpieces. In this regard there is no need for further elaboration. Normal chucks have in common that the plurality of clamping jaws—normally three or four—are simultaneously adjusted manually, hydraulically or electrically and always have the same distance to the axis of rotation.
The present invention in particular relates to a clamping device for centric clamping of workpieces with a shape deviating from perfect roundness or of rectangular workpieces or of round workpieces with defects in shape.
It is apparent that such a clamping task cannot be accomplished with a normal three- or four-jaw chuck.
To this effect, chucks with four clamping jaws are known from the state of the art, wherein in each case two opposite clamping jaws have a common drive and can be adjusted simultaneously. The method used in this context is based on a pair of jaws, opposite to one another by 180 degrees, being initially moved toward the workpiece and centering the workpiece on the common axis toward the axis of rotation. Subsequently, the pair of jaws shifted by 90 degrees is moved toward the workpiece and also performs the task of ensuring the centering in the direction of the axis of rotation. After this centering process, the clamping pressure necessary for lathing is applied. This method has proven successful in connection with clamping using clamping jaws even though the workpiece is not centered exactly on the axis of rotation and the clamping pressure on the four jaws is normally slightly different. Building on this method, in order to improve and accelerate the clamping process, hydraulically operated chucks are known instead of the mechanic drive, wherein an essential difference to manual clamping is based on the double-stroke cylinder.
What is disadvantageous with this solution is, however, that centric clamping with a uniform clamping pressure on all clamping jaws is still not possible, in particular in connection with defects in shape. There is no real possibility for reliable centric clamping of blanks with shape deviations. In particular in connection with unroundnesses, it follows inevitably that a jaw does not abut on the workpiece or that the pairs of jaws shifted by 90 degrees abut on the workpiece with different clamping pressures.
In particular the attempt at centric clamping of the workpiece leads to a significant clamping effort when several concentricity checks and subsequent recentering of the workpiece become necessary. Furthermore, the chuck has to be rotated each time to that end, or the machine operator has to go around the face chuck when working at a big rotary table.
For the application under consideration, the workpieces are therefore normally clamped on so-called face chucks. In this respect, the clamping jaws of the chuck—normally, there are four of them—can be adjusted separately. In this respect, the clamping jaws are mounted directly or indirectly in guides with a radial orientation. In this respect, it is possible to clamp workpieces with largely any geometry.
A well-known embodiment comprises four guides on the upper section of the face chuck, which are arranged at an angle of 90 degrees in relation to one another and centric in relation to the axis of rotation. In these guides, basic jaws are mounted slidably. These basic jaws serve to hold the actual clamping jaws, which are normally screwed onto the basic jaws. The basic jaws are each equipped with a spindle drive and can thus be adjusted separately. In this respect, the basic jaws cover the whole clamping range of the face chuck. This design can be compared with a typical chuck, except that each clamping jaw is adjusted separately.
In another well-known embodiment, the face chuck surface is equipped with four detent devices shifted by 90 degrees in relation to one another. These detent devices are also centric in relation to the axis of rotation. So-called jaw boxes are mounted here, wherein these can be adjusted and fixed radially by means of the detent devices. These jaw boxes are each equipped with a guide, which is also radial in relation to the axis of rotation. In these guides, basic jaws are mounted slidably, which serve to hold the clamping jaws. Here, each individual basic jaw can also be adjusted by means of a spindle drive. With this embodiment, the jaw stroke is restricted to a short path. Due to the adjustability of the jaw boxes, a large clamping range is also covered with such a chuck.
Consequently, it is not at all guaranteed with this clamping method that all jaws abut on the workpiece with the same clamping pressure. In particular in connection with clamping thin-walled workpieces or rings, clamping errors can occur. A particularly problematic situation arises when thin-walled workpiece blanks are not perfectly round. This can lead to misjudgements on the part of the machine operator when concentricity checks at the workpiece reveal constant deviations. In this respect, with this clamping method the quality of the workpiece to be processed is highly dependent on the experience and skills of the machine operator.
Furthermore, so-called pendulum jaws or system pendulum apparatuses are known from the state of the art, wherein two clamping jaws are swivelably mounted to a basic support in each case. Even though this solution allocates a uniform clamping pressure to two clamping jaws at a time in an advantageous fashion, it is almost impossible to clamp unround workpieces. As soon as the opposite clamping jaws of two pendulum jaws abut on the workpiece fixing it, it is almost impossible to displace the workpiece by means of the other transverse clamping jaws. Consequently, the other two pairs of pendulum jaws can only be adjusted toward the workpiece without allowing an exact centering in the second orientation thereof, however. It seems to suggest itself that, in order to ensure the retention force required, only one pendulum jaw belonging to the second pair of pendulum jaws can abut rigidly during the clamping process whereas at least one pendulum jaw cannot generate enough clamping pressure. A clamping process at staggered intervals with the jaw pairs shifted at an angle of 90 degrees, which would be necessary for workpieces which are not round, therefore prevents the use of pendulum jaws with the purpose of enabling centric clamping of workpieces without any deformations arising.